I am an avid cyclist. I would be happy to take any "natural" substance that helped my performance, but did not harm my body. Today, a well know professional pulled out of the Tour of Utah due to a positive finding of synthetic testosterone in a blood sample from July. (Cycling now tests riders randomly, even when they are not racing. They can just show up at your door any day and ask for a test. This is why I believe cycling is getting cleaner.)
Tommy Danielson believes that the positive sample was from a contaminated supplement he was taking . (Let's assume that this former doper, who rode with Lance Armstrong, has reformed as is not lying. I tend to believe him.) Why was he taking a supplement? He likely was taking something that someone recommended as a legal way to improve his performance.
However, there is little evidence that any sports supplements actually improve performance. I haven't reviewed the literature recently. But the last time I checked, the only two supplements that had some minor evidence were: Vitamin C for prevention of colds in endurance athletes and creatine for some body builders and vegetarians who did not get it from their diet.
The supplement industry is basically unregulated. It is a multi-billion dollar industry, but a law in the United States (DSHEA) prevents the Food and Drug Administration from regulating it. Basically, the supplement companies can make any claims they want, as long as they do not say it prevents a disease. Additionally, they are not subject to inspections or quality controls, so contaminants can get into them. (I'm not clear how synthetic testosterone gets into them, but maybe it was made in the same facility as testosterone.)
So why risk taking a supplement that is not going to help you? It might get you disqualified from a race, affect your career, and could even harm you. If you have to take one, and there is solid randomized controlled evidence for it, get it tested or use one certified by USP.
What if you are not an athlete? Maybe you just hear you should take glucosamine, Vitamin D, or Vitamin C to be healthy? As I've said before, multivitamins are not helpful, and may even cause harm. The evidence is clear. STOP taking them.
One of the last hopes of the Vitamin industry was Vitamin D. There was controversy for many years with many recommending its use, despite an National Academy of Medicine report advising against. Now we finally have a good study that supplementing with Vitamin D does not help women with mild "deficiency".
Many people tell me that eating healthy is too expensive. Yet Americans spend billions of dollars a year on supplements that do little to nothing. Let's put that money into buying healthy food. Until the supplement industry can produce high quality randomized controlled trials showing that a product works, I suggest staying away from them.